Although environmental factors such as precipitation and temperature are recognized as influencing pollen production, the impact of rising atmospheric carbon dioxide concentration ([CO2]) on the potential growth and pollen production of hay-fever-inducing plants is unknown. Here we present measurements of growth and pollen production of common ragweed (Ambrosia artemisiifolia L.) from pre-industrial [CO2] (280 mol mol–1) to current concentrations (370 mol mol–1) to a projected 21st century concentration (600 mol mol–1). We found that exposure to current and elevated [CO2] increased ragweed pollen production by 131 and 320%, respectively, compared to plants grown at pre-industrial [CO2]. The observed stimulations of pollen production from the pre-industrial [CO2] were due to an increase in the number (at 370 mol mol–1) and number and size (at 600 mol mol–1) of floral spikes. Overall, floral weight as a percentage of total plant weight decreased (from 21% to 13%), while investment in pollen increased (from 3.6 to 6%) between 280 and 600 mol mol–1 CO2. Our results suggest that the continuing increase in atmospheric [CO2] could directly influence public health by stimulating the growth and pollen production of allergy-inducing species such as ragweed.

If intraspecific variation to rising atmospheric CO2 exists in soybean [Glycine max (L.) Merr.], such variation could be used to select for optimal, high‐yielding cultivars. To quantify the range and determine the basis for variation in seed‐yield with increasing CO2, eight ancestral and one modern soybean cultivar differing in determinacy, maturity group, and morphology were grown to reproductive maturity at two CO2 partial pressures, 40 Pa (ambient) and 71 Pa (elevated). Experiments were replicated three times in temperature controlled glasshouses during 1998 and 1999. Although all cultivars showed a significant increase in seed yield with elevated CO2,(∼40%) Mandarin, an ancestral indeterminate cultivar, showed a greater relative response of seed yield to increased CO2 than did all other cultivars (∼80%). The observed variation in seed yield response to CO2 was not correlated with any vegetative parameter. At maturity, significant correlations in the relative response of seed yield to CO2 were observed for both pod weight per plant and seed weight from branches. The later observation suggests that the sensitivity of seed yield response to CO2 was associated with plasticity in the ability to form new seed in axillary branches in a high CO2 environment. Genotypic differences in the seed yield response among existing ancestral soybeans suggests that sufficient germplasm is available for breeders to begin selecting lines which maximize soybean yield in response to increasing atmospheric CO2

The growth characteristics of six and the reproductive development of five soybean [Glycine max (L.) Merr.] cultivars were examined at 39 Pa (ambient) and 70 Pa (elevated) CO2 partial pressures in temperature-controlled glasshouses. Significant intraspecific variation for both growth and seed yield in response to elevated CO2 was observed among the cultivars. At elevated CO2, total biomass increased an average of 42% at the end of the vegetative stage, while average seed yield increased by only 28%. No changes in % protein or % oil were observed for any cultivar at elevated CO2, relative to ambient CO2. The relative enhancement of either vegetative or reproductive growth at elevated CO2 was not correlated with changes in the absolute or relative increase in single leaf photosynthetic rate among cultivars at elevated CO2. For soybean, the greatest response of seed yield to elevated CO2 was associated with increased production of lateral branches, increased pod production or increased seed weight, suggesting different strategies of carbon partitioning in a high CO2 environment. Data from this experiment indicates that differences in carbon partitioning among soybean cultivars may influence reproductive capacity and fecundity as atmospheric CO2 increases, with subsequent consequences for future agricultural breeding strategies.

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